Orexin/hypocretin signaling is mediated by two receptors and two peptide agonists. The peptides (orexin-A and orexin-B) are cleavage products of the same gene, pre-pro orexin. In the central nervous system, neurons producing pre-pro orexin are found solely in the perifornical nucleus, the dorsal hypothalamus, and the lateral hypothalamus (Peyron et al., 1998, J. Neurosci. 18: 9996-10015). Orexigenic cells in these regions project to many areas of the brain, extending rostrally to the olfactory bulbs and caudally to the spinal cord (Van den Pol, 1999, J. Neurosci. 19: 3171-3182).
The orexins bind to two high affinity receptors, referred to as orexin-1 and orexin-2 receptors. Orexin-1 and orexin-2 receptors are G-protein-coupled, seven transmembrane receptors that share over 64% amino acid sequence identity with one another. Both receptors are generally excitatory, the common cellular response to orexin-induced receptor activation being increases in intracellular calcium. Homology between the species orthologs is high and there are no known pharmacological differences. Orexin-A and -B are usually considered equal ligands for orexin-2 receptor but orexin-B is thought to be 5- to 100-fold weaker ligand than orexin-A at the orexin-1 receptor (Sakurai et al., 1998, Cell 92: 573-585; Ammoun et al., 2003, J. Pharmacol. Exp. Ther. 305: 507-514).
Many regions of the brain have fairly selective expression of the orexin-1 or orexin-2 receptors (Marcus et al., 2001, J. Comp Neurology 435, 6-25; Trivedi et al., 1998, FEBS Letters, 438, 71-75). Orexin-1 receptors are selective for the limbic system (bed nucleus of the stria terminalis and amygdala), cingulate cortex and noradrenergic neurons in the locus coeruleus. Conversely, the orexin-2 receptor is almost the exclusive orexin receptor in the histaminergic neurons in the tuberomammilary nucleus which play a critical role in wake promotion; in paraventricular neurons and the parabrachial nucleus. In other brain regions like the dorsal raphe, the ventral tegmental area or the prefontal cortex both receptors are coexpressed.
The broad CNS distribution of cells producing orexin, as well as cells expressing the orexin receptors, suggests involvement of orexin in a number of physiological functions, including feeding and metabolism, regulation of wakefulness and sleep, sympathetic activation and stress response (de Lecea, 2012, Progress in Brain Research, 198, 15-24; Kukkonen, 2013, Am J. Physiol. Cell Physiol., 304, C2-C32). Orexin also plays a key role regulating motivation and reward associated with food intake and with drugs of abuse (Mahler et al., 2012, Progress in Brain Research, 198, 79-121).
Several lines of evidence indicate that the orexin system is an important modulator of arousal. Rodents administered orexin intracerebroventricularly spend more time awake (Piper et al., 2000, J. Neurosci. 12: 726-730. Orexin-mediated effects on arousal have been linked to orexin neuronal projections to histaminergic neurons in the tuberomammillary nucleus (Yamanaka et al., 2002, Biochem. Biophys. Res. Comm. 290: 1237-1245). Rodents whose pre-pro orexin gene has been knocked out, or whose orexigenic neurons have been killed, display altered sleep/wake cycles similar to narcolepsy (Chemelli et al., 1999, Cell 98: 437-451; Hara et al., 2001, Neuron 30: 345-354). Dog models of narcolepsy have been shown to have mutant or non-functional orexin-2 receptors (Lin et al., 1999, Cell 98: 365-376). Orexin signaling as a target for sleep-promoting therapies was further validated clinically by findings of attenuated orexin levels and loss of orexinergic neurons in human narcoleptic patients (Mignot et al., 2001, Am. J. Hum. Genet. 68: 686-699; Minot & Thorsby, 2001, New England J. Med. 344: 692) or, in rare cases, to mutations in the orexin-2 gene (Peyron et al., 2000, Nature Med. 6: 991-997). Disorders of the sleep-wake cycle are therefore likely targets for orexin-2 receptor modulator activity. Examples of sleep-wake disorders that may be treated by agonists or other modulators that up-regulate orexin-2 receptor-mediated processes include narcolepsy, jet lag (sleepiness) and sleep disorders secondary to neurological disorders such as depression. Examples of disorders that may be treated by antagonists or other modulators that down-regulate orexin-2 receptor-mediated processes include insomnia, restless leg syndrome, jet lag (wakefulness) and sleep disorders secondary to neurological disorders such as mania, schizophrenia, pain syndromes and the like.
Evidence has accumulated to demonstrate a clear involvement of orexin signaling in reward pathways associated with drug dependence (Mahler et al., 2012, Progress in Brain Research, 198, 79-121). Orexinergic neurons send projections to the ventral tegmental area and other brain regions involved in reward processing. Orexin ligands mediate reward behavior, and antagonizing these effects with a selective orexin-1 receptor antagonist in various preclinical model of addiction has suggested that these actions are mediated through orexin-1 receptor. Specifically, a selective orexin-1 antagonist attenuates morphine conditioned place preference and reinstatement (Harris et al., 2005, Nature, 437, 556-5599; Narita et al., 2006, J. Neurosci., 26, 398-405; Harris et al., 2007, Behav Brain Res, 183, 43-51), stress-induced cocaine reinstatement, cocaine-induced behavioral and synaptic plasticity (Borgland et al., 2006, Neuron, 49, 589-601), and intake and cue and stress-induced reinstatement of ethanol (Lawrence et al., 2006, Br J Pharmacol, 148, 752-759), in addition to attenuating precipitated morphine withdrawal (Sharf et al., 2008, Biol Psychiatry, 64, 175-183) and nicotine self-administration (Hollander et al., 2008, Proc Natl Acad Sci USA., 105, 19480-19485). Another recent study has also suggested a role for OX2R (Shoblock et al., 2011, Psychopharmacology, 215, 191-203).
Orexin's role in more complex emotional behavior is also emerging (Johnson et al., 2012, Progress in Brain Research, 198, 133-161). Changes in orexin levels in patients with panic and posttraumatic stress disorders have been noted as have changes in the prevalence of anxiety behaviors in narcoleptic patients (Johnson et al., 2010, Nature Medicine, 16, 111-115; Fortuyn et al., 2010, General Hospital Psychiatry, 32, 49-56; Strawn et al., 2010, Psychoneuroendocrinology, 35, 1001-1007). Lactate infusion or acute hypercapnia, which causes panic in humans, and are used as an animal model of panic, activates orexin neurons in the perifomical hypothalamus. This activation correlates with anxiety in the social interaction test or open field test. Blocking orexin signaling with either siRNA or selective orexin-1 receptor antagonists attenuates panic-like responses to lactate (Johnson et al., 2010, Nature Medicine, 16, 111-115; Johnson et al., 2012, Neuropsychopharmacology, 37, 1911, 1922).
Cerebral spinal fluid (CSF) levels of orexin are lower in depressed or suicidal patients, and the level of orexin inversely correlates with illness severity (Brundin et al., 2007, European Neuropsychopharmacology, 17, 573-579; Salomon et al., 2003, Biol Psychiatry, 54, 96-104). A positive correlation between orexin-1 receptor mRNA in the amygdala and depressive behavior in the forced swim test in mice has been reported (Arendt, 2013, Behavioral Neuroscience, 127, 86-94).
The orexin system also interacts with brain dopamine systems. Intracerebroventricular injections of orexin in mice increase locomotor activity, grooming and stereotypy; these behavioral effects are reversed by administration of D2 dopamine receptor antagonists (Nakamura et al., 2000, Brain Res. 873: 181-187). Therefore, orexin receptor modulators may be useful to treat various neurological disorders; e.g., agonists or up-regulators to treat catatonia, antagonists or down-regulators to treat Parkinson's disease, Tourette's syndrome, anxiety, delirium and dementias.
Orexins and their receptors have been found in both the myenteric and submucosal plexus of the enteric nervous system, where orexins have been shown to increase motility in vitro (Kirchgessner & Liu, 1999, Neuron 24: 941-951) and to stimulate gastric acid secretion in vitro (Takahashi et al., 1999, Biochem. Biophys. Res. Comm. 254: 623-627). Orexin effects on the gut may be driven by a projection via the vagus nerve (van den Pol, 1999, supra), as vagotomy or atropine prevent the effect of an intracerebroventricular injection of orexin on gastric acid secretion (Takahashi et al., 1999, supra). Orexin receptor antagonists or other down-regulators of orexin receptor-mediated systems are therefore potential treatments for ulcers, irritable bowel syndrome, diarrhea and gastroesophageal reflux.
Body weight may also be affected by orexin-mediated regulation of appetite and metabolism. Some effects of orexin on metabolism and appetite may be mediated in the gut, where, as mentioned, orexins alter gastric motility and gastric acid secretion. Orexin antagonists therefore are likely to be useful in treatment of overweight or obesity and conditions related to overweight or obesity, such as insulin resistance/type II diabetes, hyperlipidemia, gallstones, angina, hypertension, breathlessness, tachycardia, infertility, sleep apnea, back and joint pain, varicose veins and osteoarthritis. Conversely, orexin agonists are likely to be useful in treatment of underweight and related conditions such as hypotension, bradycardia, amenorrhea and related infertility, and eating disorders such as anorexia and bulimia.
Intracerebroventricularly administered orexins have been shown to increase mean arterial pressure and heart rate in freely moving (awake) animals (Samson et al., 1999, Brain Res. 831: 248-253; Shirasaka et al., 1999, Am. J. Physiol. 277: R1780-R1785) and in urethane-anesthetized animals (Chen et al., 2000, Am. J. Physiol. 278: R692-R697), with similar results. Orexin receptor agonists may therefore be candidates for treatment of hypotension, bradycardia and heart failure related thereto, while orexin receptor antagonists may be useful for treatment of hypertension, tachycardia and other arrhythmias, angina pectoris and acute heart failure.
From the foregoing discussion, it can be seen that the identification of orexin receptor modulators, will be of great advantage in the development of therapeutic agents for the treatment of a wide variety of disorders that are mediated through these receptor systems.